1. Field of the Invention
The present invention relates generally to a toner image fixing apparatus for thermally fixing a powdered toner image formed on an image recording material and, more particularly, to fixing rollers, that is, a thermal roller and a cooperating pressure roller, for fixing a powdered toner image onto an image recording sheet a in electrophotographic equipment, photocopying machines, image forming equipment, and the like.
2. Description of the Prior Art
In general, electronic photocopying machines form a duplicate of an original image by first reflecting light from the original onto a sensitized, electrostatically charged drum to form an electrostatic latent image. Powdered toner is attached onto the electrostatic latent image on the charged drum to visualize the latent image, and the toner image is transferred onto a toner-receiving sheet. The image is then thermally fixed by a fixing apparatus.
In the fixing apparatus of conventional copying machines, a toner-receiving sheet to which the toner image is transferred is fed through a nip defined between a heated roller and a cooperating pressure roller so that the toner image is permanently fixed onto the copy sheet as a function of the heat provided by the heated roller and the compression force provided by the pressure roller. In conventional toner image fixing apparatus, a hollow thermal roller is typically heated by a lamp (e.g., a halogen lamp) co-axially located within thermal roller with the radiation from the lamp heating the interior of the roller and the heat energy transferred to the exterior of the roller by conduction. This method of heating the thermal roller presents a number of disadvantages that result in a less than optimum thermal efficiency. More specifically, the preheating time is undesirably long because of the need to raise the temperature of the entire thermal roller by radiant energy, and it is difficult to quickly control the surface temperature of the thermal roller because of the thermal conduction time delays. If the surface temperature of the roller is not set to an appropriate value, the thermal roller is excessively heated at the opposite ends and can cause thermal deterioration in adjoining or adjacent elements, or, if the thermal roller is not heated to a sufficiently high temperature, the toner image will not be fixed on the toner receiving sheet in a permanent and stable manner.
As an alternative to a lamp-heated thermal roller, a resistive heat-generating layer can be used to provided the heat energy necessary to perform the fixing operation. The heat-generating layer is formed integrally with the thermal roller and generates heat in response to the conduction of electricity therethrough. While a heat-generating layer allows comparatively rapid control of the surface temperature of the thermal roller, an uneven temperature distribution along the length of the roller can occur when many sheets of copying paper of different widths are fixed continuously between the thermal roller and the pressure roller. The uneven temperature distribution typically results in a markedly lowered temperature at the opposite ends of the thermal roller making it impossible to achieve a uniformly acceptable fixing temperature alon the entire length of the thermal roller.